[mips] Rename function in MipsAsmParser.

parseDirectiveWord is a generic function that parses an expression which
means there's no need for it to have such an specific name. Renaming it to
parseDataDirective so that it can also be used to handle .dword directives[1].

[1]To be added in a follow up commit.

No functional changes.

llvm-svn: 204818